The Balancing Act – Rotor Stability

When designing rotating equipment, it is extremely important to take into account the types of unbalance that can occur. Forgetting this step can result in vibrations that lead to damage of the rotating parts, increasing maintenance costs and lowering efficiency. Currently, if a rotating part already vibrates or makes any noises, maintenance engineers rely on OEMs (Original Equipment Manufacturer) or third parties services companies to conduct balancing services.

Types of Unbalances

Figure 1: Static and Couple Forms of Unbalance

The three types of unbalances to consider are static, couple and dynamic. Static unbalance (Figure 1) occurs when a mass at a certain radius from the axis of rotation causes a shift in the inertia axis. Couple unbalance, usually found in cylindrical shapes, occurs when two equal masses positioned at 180 degrees from each other cause a shift in the inertia axis, leading to vibration effects on the bearings. Lastly and most common, dynamic unbalance occurs when you have a combination of both static and couple unbalance.

Balancing Methods used Today

Figure 2 High-Speed Balance

Two of the most popular balancing methods that are performed on existing turbine rotors are low-speed balancing and high-speed balancing methods. A low-speed balance helps determine wear and tear on rotor under minimum speed conditions. This method is generally used by companies who lack access to a high-speed balance cell. A high-speed balance (Figure 2) allows the test be run at or passed operating speed, and even though it is more expensive, it tends to be a more accurate method of testing. The primary reason why  more end users are opting for this type of balance, for both turbine rotors and power generator fields, is that all major components can be tested under rigorous conditions. By doing this, the full operating lives of these components can accurately be determined.

Want to make sure you’re taking into account unbalances when designing rotors in rotating equipment? Look into our rotordynamics software capabilities 


Reciprocating Machinery Dynamics by Abdulla S. Rangwala – Chapter 9

The Importance of Turbulence Modelling

What is the importance of turbulence modelling in capturing accurate 3D secondary flow and mixing losses in turbomachinery? An investigation on the effect of return channel (RCH) dimensions of a centrifugal compressor stage on the aerodynamic performance was studied to answer this question by A. Hildebrandt and F. Schilling as an effort to push turbomachinery one step further.

W. Fister was among the first to investigate the return channel flow using 3D-CFD. At that time the capability of commercial software was not extended and any computational effort was limited by the CPU-capacity. Therefore, only simplified calculations that included constant density without a turbulence model (based on the Prandtl mixing length hypothesis) embedded in in-house code, were performed.

Although separated flow without a predominant flow direction could not have been calculated, the method indicated separated flow regions with relatively accurate precision, and it predicted the magnitude of loss coefficients to a higher degree than experimental data. The study was further
simplified using incompressible flow, and an axial U-turn inlet flow.AxCFD

The biggest drawback of using inverse methods for return channel design refers to the question of appropriate flow distribution across the RCH surface. Furthermore, flow separation cannot be predicted with the help of singularity methods. In order to circumvent the problem of predicting flow separation, nowadays compressible viscous 3D-CFD applied with different highly complex turbulence modeling is the state of the art even at the conceptual stage of the design.

Hildebrandt and F. Schilling analyzed three different centrifugal stages regarding the return channel system performance. All three stages featured the same impeller type, two of them being applied with a 3D-RCH at different flow coefficient and one impeller being applied with a 2D-RCH system. The 3D-RCH stage featured both CFD calculated and measured superior aerodynamics over the 2D-RCH stage regarding the overall performance as well as regarding the outlet flow angle. The comparison between the measured and the CFD-predicted performance showed agreement both when it comes to overall performance (efficiency, pressure rise coefficient) and also regarding detailed flow field (outlet flow field). The 3D secondary flow and mixing losses of the entire domain downstream the vaneless diffuser were either underestimated or overestimated by the CFD-calculations, depending on the turbulence modeling and the impeller fillet radii-modeling which affects the RCH-inlet flow conditions. The effect of fillet radii-modeling on the RCH-exit flow angle spanwise distribution was found to be significant in order to better match the experimental results.

It is worth noting that the rather simple Spalart–Allmaras turbulence model provided better agreement with the measured RCH-exit flow angle distribution than the more sophisticated k-epsilon model, which on the other hand, outputted a closer fit with the measured surface vane pressure distribution. Regarding the RCH total pressure loss distribution, none of the models showed a perfect agreement with the measurement data.

Moreover, the incident losses of the 3D-RCH system seemed to play a minor role within the overall RCH-loss which is significantly dominated by the 3D-secondary losses.

Interested in learning more? Check out AxSTREAM and AxCFD!

[1] A. Hildebrandt and F. Schilling, 2017 “Numerical and Experimental Investigation of Return Channel Vane Aerodynamics With Two-Dimensional and Three-Dimensional Vanes”, Journal of Turbomachinery Vol. 139 / 011010-1

[2] Fister, W., Zahn, G., and Tasche, J., 1982, “Theoretical and Experimental Investigations About Vaneless Return Channels of Multi-Stage Radial Flow Turbomachines,” ASME Paper No. 82-GT-209.

The Future of Turbocharger Technology

­One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio.  Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1).  This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.

Ratio visualization
Figure 1 – A/R Ratio Visualization

For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds.  This results in a more responsive engine and overall higher boosts at lower RPMs.  However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system.  This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine.  The same effect is present for larger turbochargers, only in reverse.  They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.

In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo.  One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series.  The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.

Other technologies are very common for overcoming this problem without the use of two separate turbos.  One design is the twin-scroll turbocharger, which works like a pair of turbochargers connected in parallel.  Instead of using two separately sized turbochargers, the twin-scroll design has two exhaust-gas inlets and two nozzles feeding into a single turbocharger.  Another design, the variable geometry turbocharger, uses internal vanes within the turbocharger to dynamically alter the A/R ratio of the system at different rev levels.  The variable geometry turbocharger (VGT) displays exceptional performance at the low and high RPM ranges, however, it requires the use several moving parts and exotic materials in order to handle the high exhaust temperatures present in petroleum engines.  For this reason, the variable geometry turbocharger is generally only used in diesel engines due to the relatively low exhaust temperatures in comparison.  Twin-scroll turbochargers, while more commonly used in gasoline applications, do not have the variability of the VGT and are not as efficient over the full rev range of the vehicle.

Figure 2 – BorgWarner Twin-Scroll Variable Geometry Turbocharger Concept

BorgWarner has recently showcased a new turbocharger concept that makes use of both technologies discussed above.  The “Twin-Scroll Variable Geometry Turbocharger” has a valve that can redirect airflow from just a single scroll to an airflow completely split between two scrolls.  It allows for the any range in between these extremes much like the VGT; however, it only uses only one valve as opposed to the several vanes needed in the VGT.  It derives the same exhaust separation idea as the twin-scroll while allowing for the variability of the VGT.  With all the new emissions regulations calling for increases in efficiency, it will be interesting to see if this new technology can find its way into the market.


Optimizing the Cooling Holes in Gas Turbine Blades

To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.

Figure 1: Variations of Thermal Efficiency with TIT [1]
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of  the gas turbine.

With this in mind, during  the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.

Different cooling methods that are employed depend on the extent of the cooling required. The cooling flow passes through several loops internally and is then ejected over the blade surface to mix with the main flow. The mixing of the cooling flow with the main flow alters the aerodynamics of the flow within the turbine cascade. The cooling flow that is injected into the main flow needs to be optimized, not only in terms of thermodynamic parameters, but also  in terms of the locations to ensure the turbine vanes and blade surfaces are maintained well below the melting surface. The spacing between the holes, both in horizontal and vertical direction, affects not only the surface temperature of the blade, but also the strength of the blade and its overall life.

Performing a 3D analysis for optimizing the flow, spacing, and location of cooling flow is computationally expensive. A 1D flow and heat network simplifies the task of not only arriving at the optimal configuration of cooling holes and location, but also in aerothermal design of the gas turbine flow path and generation of the optimized 3D blades with reduced overall design cycle time. Designers are faced with the challenge of simplifying the complex 3D cooled blade and accurately modelling it. AxNET, the module for 1-dimensional flow and heat transfer provides designers options to not only use the different components and models inbuilt in the module but also customize to represent the 3D blade as accurately as possible in a simplified approach

Gas Turbine 3D Part 1

Simplified representation of the 3D gas turbine blade cooling holes using 1D flow and heat networt

To learn more about how AxSTREAM Net can help you optimize the cooling holes in gas turbine blades, please contact us at or


[1] Amjed Ahmed Jasim Al-Luhaibi and Mohammad Tariq, “Thermal Analysis of cooling effect on gas turbine blade” International Journal of Research in Engineering and Technology, eISSN: 2319-1163, pISSN:2321-7308

Exotic Turbomachinery – Viscous Disc Pumps

Turbomachinery can be divided into two main groups. Group one consists of machines that perform work on the fluid, requiring energy and increasing its pressure, such as compressors, pumps, and fans. Group two consists of those that extracts energy from the fluid flowing through it – for example, wind, hydro, steam, and gas turbines.

Pumps specifically are devices whose purpose is to move fluid at a constant density, increasing its kinetic energy and its pressure while consuming energy in the process. We are quite used to seeing centrifugal and axial pumps, as they are the most common configurations.  However, more complicated designs have been tested and developed throughout the history of fluid machinery.

VTeslaiscous disk pumps are one such design – inspired by the concept of Tesla Disc Turbine, and conceived and developed in 1913. The key feature of these pumps is the bladeless design, consisting of multiple parallel rotating discs within a casing. Additionally, its operating principle is based in the so-called boundary layer effect. The main difference from a conventional turbine design is that there is no impingement of the rotating parts on the fluid.

As shown in figure 1, the nozzle is inserting the fluid to the discs’ edge, which is dragged by the moving fluid through the viscosity and fluid adherence to the discs’ surface. After energy is transferred to the discs, the fluid gets extracted by the center exhaust.

The same set of discs and a slightly different shape of casing/volute can be used as a pumping device, which is also called a boundary layer disc pump.

Even if Tesla’s turbine design didn’t undergo further development, the disc pump design found its purpose in highly demanding pumping applications.


In the last 30 years this type of pump has found its niche in high viscosity fluids and low Reynolds number flows. For instance, common applications include the pumping of crude oil, sludge, food pulps and wastewaters. They offer excellent pumping capacity for fluids with enthralled gases or delicate solid particles.

Due to its operating principles, flow is similar to an ordinary pipe with parabolic velocity profiles and stationary layers of fluid (relative to the rotating discs). This allows enthralled gases or solid particles to remain located in the core of the flow with no contact to the discs. As such, the reliability and lifetime for these pumps is very high, as the moving parts show little to no deterioration and require minimal maintenance.


In these conditions, viscous pumps show much higher efficiency despite the lower level of complexity when compared to their conventional counterparts with centrifugal design. Less complexity means lower maintenance and lower costs.

Simplicity of the design in these pumps also shows promising application in microfluidics with different miniaturized designs for drugs transportation and biomedical applications, such as blood transportation with no damage to plasma particles. Also in these operating conditions with low Reynolds flow, disc pumps have shown efficiencies as high as 95%, much higher than a centrifugal or displacement counterpart. Different designs have been investigated and proposed, involving 1, 2 or more bladed discs, depending on the application and size.

Despite of the advancements in the design of turbomachinery, and the increase in computing power creating more and more complex fluid dynamics analyses, viscous disc pumps remain a unique case that shows that complexity and over-development do not necessarily equal higher efficiency and performance.


“Single-Disk and Double-Disk Viscous Micropump” – D. Blanchard, P. Ligrani and B. Gal – Sensors  and Actuators A-Physical, 122, 149-158, 2005

“Analytical and experimental modeling of a viscous disc pump for MEMS applications” – Marco D. C. Oliveira and José C. Páscoa

The Origin of Rotordynamics

Rotordynamics, the study of vibrational energy in rotors, has a rich history dating back to North America during the 1750’s. This branch of applied mechanics began with theories, but advanced quickly due to practices – starting with Mr. W.J.M Rankine in 1869 and his spinning shaft experiment. Now, decades later, we have strengthened our understanding of rotordynamics and created leading software tools, including AxSTREAM, that are able to simulate analyses to stabilize and increase the reliability of a turbomachinery.

W.J.M Rankine

Not only was W.J.M Rankine a prestigious theoretical scientist and educator, he was a main contributor in the development of rotordynamics and he contributed to thermodynamics and the development of heat engines throughout his lifetime. During his spinning shaft experiment, he concluded that beyond the shaft’s first critical speed, the shaft would be unstable simply because its shape had been bent. By not taking into consideration support damping and Coriolis force in his analysis, many engineers were left confused for almost two centuries, until Gustaf de Laval, a Swedish engineer, ran a steam turbine to supercritical speeds in the late 1880’s. Laval also introduced the use of bearings to oppose absolute motion in his machinery. As the years went by, many other engineers discovered and investigated additional phenomenons (FEM for example) that have an influence in today’s practices.
It is because of these previous innovators that companies like SoftInWay have been able to develop the advanced  rotodynamics modules that we use today.

AxSTREAM, for example is a prime example of this. The software tool saves engineering time and cost and it ensures that no destructive rotor dynamicsvibration will occur in the rotor-bearing system. It is able to perform many analyses like Static Gravity Deflection, Critical Speeds, Damped Unbalance Response, Modal and Transient. It is because of the rich history of rotordynamic development that software platforms like AxSTREAM are able to ensure the best design and performance for your turbomachinery.

Figure 2: Demostration of AxSTREAM RotorDynamics


Working with Geothermal Heat Pumps

A geothermal heat pump utilizes earth’s thermal energy as a way to manipulate temperature. This is seemingly attractive toward HVAC utilization due to the relatively high efficiency as well as economic benefit. Temperature fluctuations below ground are relatively low as earth absorbs solar energy all year round and insulates the heat underground. Taking advantage of this event, geothermal energy heat pump application for residential and commercial building uses the “underground” as a heat source/sink.

geothermal heat pumps

How does geothermal heat pump work?

A heat pump system mainly consists of a heat-pump unit, a pipeline loop functioning as a heat exchanger for a desired area (it can be horizontal, vertical or installed to an aquatic medium), and a duct – to deliver the controlled temperature flow to the consumer.

Fluid is pumped through an installed pipeline loop which transfers heat based on the season. During the hotter season (summer), heat will be absorbed from the air in the building, transferred into the ground and then cooler air will be circulated to the designated area. The contrary happens during the winter. In colder months, heat will be transferred into the fluid from the ground and collected heat will be distributed.

What are the benefit of this technology?

Every unit of electricity used by a geothermal heat pump will be transferred to 5 units of cooling or heating, consequently the geothermal heat pump is much more efficient then, for example air heat pumps. Air heat pump will move the heat from the source to an equal or higher temperature environment, making it less efficient each time the temperature increases. However, since the underground temperature is relatively stable all year round, geothermal heat pumps don’t encounter the same event. Additionally, since the heat can be transferred to any kind of fluid, geothermal heat pumps can also be used as a main water heating source. Since there is no “burning” event during the engineering process with heat pump, there is no carbon monoxide or excess product from this system, making it environmentally friendly as well.

Study your energy cycle using AxCYCLE!


Performance Effects of Axial Turbines & Compressors Due to Roughness Variations

As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate.  Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness.  Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.

gas turbine blade
Figure 1 – Gas Turbine Blade and Annulus Surface Wear (Source PowerMag)

There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces.  Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow.  Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces.  Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.

Some of these mechanisms, however, are more prevalent in higher temperature systems.  Hot corrosion, for example, occurs at a temperature range of around 730 to 950°C due to the chemical interaction of molten salts on the component’s surface.  In this case, high temperature gas turbines are most notably affected by this phenomenon.  Even with protective cooling flow systems, most gas turbines start to lose efficiency early in their life cycles.  For this reason, it is important to analyze the future performance of the machine before it is put into operation.  Attributing the necessary roughness factors can lead to a more accurate description of the performance of a machine over time.  Using the AxSTREAM software platform, these roughness factors can be implemented into a design to compare what a turbomachine might experience 5 or 10 years into its lifecycle as opposed to at its birth.  Furthermore, a user could model the effect of particular surface coating and polishing techniques and analyze whether the increases in efficiency outweighs the investment costs.

Streamline - Axial Turbine
Figure 2 – Streamline Calculation Stage for Axial Turbine in AxSTREAM

By assigning different surface roughness grades in AxSTREAM, one can analyze a number of parameters of interest such as efficiency and power output.  Using the streamline/meanline calculation module, (seen to the left), the user is able to seamlessly overlay a number of different performance factors and curves due to the prescribed changes in material roughness.  With increases in surface roughness, laminar to turbulent transitions will occur much sooner and will result in substantial losses in the machine.  Because of this, it is crucial to investigate what kind of investments and preventative measures need to be made to ensure proper treatment of a machine’s surface.

If you’re interested in learning more about our integrated software platform for turbomachinery applications, please read here.


Discussion – Alien Signal or Radio Noise: Leveraging Turbomachinery

The Internet practically exploded early yesterday morning with talk of an extraterrestrial discovery after a signal was detected by a Russian telescope. The star in question, HD 164595 located a vast 95 light years away, sent out a strong radio spike that was picked up and sparked a boom of excitement. According to an article published by National Geographic, however, this signal may not be what it was first interpreted as.

Astronomers have pointed their radio telescopes towards the stars for over half a century, hoping to catch a glimmer of life beyond this planet. Short of a futuristic rocket ship, these telescopes seem to be the best bet for catching a peak of something out of this world. That is a main causStarse as to why this discovery is so tantalizing to both scientists and the rest of us earthlings. However, after further investigation, neither the Allen Telescope Array, commanded by the SETI (the Search for Extra-Terrestrial Intelligence) Institute, nor the Green Bank Telescope, used by the Breakthrough Listen project, turned up additional signals or observations.

Another issue that has risen according to this article is that the signal did not repeat and could have been caused by something else. A source on Earth, such as a faulty power supply, military transmission, or arcing electrical fence for example. Another possible explanation could be that gravity from another object in space amplified a weaker signal. That being said, it would appear that HD 164595 is similar in many ways to our sun. It is composed of the same ingredients, is approximately the same age and has at least one planet in its orbit. This would suggest that theoretically, it would be plausible for life to exist within this system.

It’s safe to say that most of us hope to have an answer to the question of whether there is civilization somewhere in the sweeping unknown. Here at SoftInWay, we may not have a futuristic rocket ship stashed away, but we do hope to one day play a role in assisting these scientists find theiRocket Engine Thingyr answer. Perhaps, one day, SoftInWay can design and provide guidance on the rocket engine turbopump that will help us soar into new discoveries and exploration. Until then, we’ll provide the tools and guidance suited towards present needs  and future goals. If your branch of work falls under the scope of aerospace, or if you find rockets as cool as we do, our upcoming webinar will interest you. It is being offered towards the end of September and provides an overview of rocket engine turbopumps, from preliminary blade design to CFD and rotor dynamics using our AxSTREAM® platform.


Feel free to click on the link below for the full article. What are your thoughts on the matter discussed in the article? Share your comments below!

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Rotor Dynamics – Importance of Fundamental Understanding & Software tools

Rotor and bearings are the most critical components of any rotating machinery. Rotor lifetime and reliability depend, first of all, on the level of rotor vibrations. In order to meet highest requirements of reliability each step of the rotor design should be based on accurate Rotor Dynamics prediction.

Rotor dynamics is the branch of engineering that studies the lateral and torsional vibrations of rotating shafts, with the objective of predicting the rotor excessive vibrations. Rotor Dynamics is different from structural vibrations analysis because of gyroscopic moments, cross-coupled forces, critical speeds, whirling effect, etc. These difference makers are all due to the rotation of the rotor assembly.


Understanding of basic rotor dynamics phenomena and the various types of problems is absolutely mandatory when designing and developing rotor-bearing systems for various applications. Fundamental approach for Rotor Dynamics analysis generally is based on the following steps:

  1.  Predict critical speeds.
  2. Determine design modifications to change critical speeds.
  3. Predict natural frequencies of torsional vibration.
  4. Predict amplitudes of synchronous vibration caused by rotor unbalance.
  5. Predict threshold speeds and vibration frequencies for dynamic instability.
  6. Determine design modifications to avoid dynamic instabilities.
  7. Calculate balance correction masses and locations from measured vibration data.

Another factor that determines accuracy of Rotor Dynamics calculation is rotor system simplification and the adequate modelling for rotor parts such as Impeller/disks, Sleeves, Balance pistons, Seals, Thrust collars, Couplings, Addition of Stiffening Due to Shrink Fits and Irregular Sections etc. (more…)