Monday, March 28, 2016

Flow Induced Vibration Part 2: Using CFD to find the forcing frequencies

Combining CFD with FEA can provide a powerful method to investigate flow induced vibration. In Part 1 of this post, the use of FEA to find the natural frequency of the spray lance was discussed. In this post, the use of CFD to find the frequencies found in the velocity fluctuations as the fluid flows across the spray lance.

The spray lance shown in the previous post was designed to be inserted into a vertical column with a turbulent airflow. An important question that had to be answered was whether the flow across the spray lance would induce vibrations within the spray lance that would lead to vibration or even to failure. Since the system was in the design phase, it was necessary to use computer simulations to investigate the expected performance of the system. A transient CFD analysis was performed to estimate the velocities that would be found within the system during normal operation.

The plot below shows the fluctuation in the average velocity along the length the spray lance as determined from the transient CFD analysis. This plot shows that significant fluctuations in the velocity can be expected and, as a result, fluctuations in the forces acting on the spray lance. 

A Fourier analysis of this velocity signal yielded a frequency spectrum for this velocity profile as shown in the figure below. This plot shows that the primary frequencies of the velocity are found below 10 Hz, with the two main frequencies at approximately 2-4 Hz.

This second look shows that there are no strong forcing frequencies found at the first or second natural frequencies of the spray lance. Like the results from the velocity analysis, these results also suggest that the configuration is unlikely to experience vibration due to the flow across the spray lance.

The method outlined in these two posts is a simple approach to looking at flow induced vibration. A complete analysis is far more complicated and takes more factors into account. This method however can be used as a screening tool to find potential problems. For example, if the velocity were at or near the critical velocity or if the frequencies in the velocity were found to be near the natural frequency of the spray lance, it would indicate that there was a strong possibility that excessive vibration would exist when the system was started. In this were indeed the case, more investigation or a redesign of the spray lance would be required.

Monday, March 21, 2016

Flow Induced Vibration Part 1: Using FEA to determine the natural frequency

In a recent project I had to design a spray lance to be inserted into a large vertical column. In an early design iteration, it was necessary to determine if the spray lance would suffer from flow induced vibration once the system was started. Since the system did not exist and there was no data about how it actually worked, a computer study of the system was performed to investigate the possibility of flow induced vibration. 

One of the useful tools in most FEA packages is the ability to determine the natural frequency of a part very quickly. Using FEA to determine the natural frequency rather than a using a hand calculation can help reduce the time and effort required to study the dynamics of a system. This is especially true if a part undergoes many design changes and the FEA package is linked to or part of the CAD package.

Knowing the natural frequency of a part is important when dealing with turbulent fluid flow or rotating machinery because if the natural frequency of a part happens to be close to the frequency of some forcing function that exists within a system the part is likely to fail at loads far below what a static analysis predicts. This occurs due to the fact that if the forcing function matches the natural frequency, the amplitude of the deflection due to the forcing function is magnified with each cycle, eventually leading to failure. This behavior is called resonance and in almost all engineering cases, is a very bad thing.

In the video example above, the first five calculated natural frequencies for the spray lance are 12.7 Hz, 14.7 Hz, 79.1 Hz, 90.4 Hz, and 218.4 Hz. 

As fluids flow across cylinders and other shapes, they can create a regular vortex pattern downstream of the shape. The frequency of these vortices can be expressed using the Strouhal Number which varies primarily as a function of shape and Reynolds number. In this case, the average fluid properties and velocity were used to calculate the Reynolds number across the spray lance to see if it falls within the zone (below Re<300,000) where vortex shedding occurs. In this case, the Reynolds number is approximately 3,600 which is in the vortex shedding zone. At this Reynolds number, the Strouhal Number is approximately 0.21. As a result, the critical velocity where the frequency of the vortex shedding equals the first two natural frequencies are 15.1 ft/s for the first natural frequency and 17.5 ft/s for the second natural frequency. The average velocity along the spray lance was found to be approximately 4.6 ft/s. At this velocity, the frequency of vortex shedding is approximately 3.9 Hz, which is well below the first natural frequency of 12.7 Hz. The fact that the average velocity is well below the first critical velocity suggests that the likelihood for vibration due to the vortex shedding is low.

Monday, March 14, 2016

Part 2: What is the basis for using ACH as a design parameter?

In a previous post I examined how the concentration of a pollutant decreased over time as a function of different ventilation rates. This examination was limited to the case where the pollutant was at some fixed level in a space and then ventilation was introduced into that space. An example of this situation would be a pollutant leaking from a pipe into a closed room and then a valve being closed which stops the flow of the pollutant and then a fan being turned on to provide ventilation to the space. While this scenario is possible, it is probably more useful to examine the case where a pollutant is being emitted at some rate and ventilation is being supplied to attempt control the level of that pollutant. For simplicity, it will be assumed that the initial concentration of the pollutant is zero. With this simplifying assumption, this case can be modeled using a relatively simple differential equation:

where   C(t) = Concentration at time t
G = Generation rate of pollutant
Q = Ventilation rate
V = Volume of the space
Dt = Change in time

This equation comes from ACGIH's book Industrial Ventilation A Manual of Recommended Practice. As with the previous case, the units for each of the parameters must be consistent. If G is given in CFM, then the time will be minutes and the volume will need to be given in cubic feet. So what impact does changing the ACH have upon the concentration of the pollutant in a space? For this example, it is assumed that the rate of pollutant generation is 1 CFM (0.5 L/s) in a room with a volume of 10,000 cubic feet (283 cubic meters), a space roughly 29’ wide x 29’ long x 12’ tall (8.8 m x 8.8 m x 3.7 m).

As in the previous case, the ACH has a dramatic impact upon the final concentration of the pollutant in the room. At small values for ACH the concentration of a pollutant increases for quite some time until a steady state concentration is reached. For example with an ACH = 0.5 the concentration continues to increase for about 10 hours until the final concentration of 12,000 ppm (1.2% by volume) is reached. Contrast this with an ACH = 4 where the final concentration of 1,500 ppm (0.15%) is reached after an hour. As ACH increases, the final steady state concentration decreases. This chart suggests that the ventilation rate can be used to control the final concentration of pollutants in a space.

It can be reasonably concluded that using the ACH as a design parameter for a ventilation system has merit. However, it is necessary to again mention that several simplifying assumptions were made in the previous analysis which can have a dramatic effect upon the performance graphs presented here. The limitations of this method will be examined in an upcoming post.

Tuesday, March 8, 2016

AIHce 2016: Understanding and Using ANSI/AIHA/ASSE Z9.2-2012

See the main conference and expo website here.

Keith D. Robinson, P.E. will be teaching a course entitled PDC 109:  Understanding and Using ANSI/AIHA/ASSE Z9.2-2012 at the upcoming AIHce conference. This course provides an in-depth look at the requirements for Local Exhaust Ventilation (LEV) systems that are set forth in this standard. It is intended for Environment Health & Safety (EH&S) personnel, facility managers, system operators, and engineers. 

Please visit my main website at for more information about Keith D. Robinson, P.E.

The System Curve, The Fan Curve, and the Operating Point Lunch and Learn

I am pleased to announce that I will be presenting a complimentary Lunch and Learn session on April 21, 2016 entitled "The System Curve, The Fan Curve, and the Operating Point". During this session, the theoretical background for how a fan and duct system interact will be presented. Following this brief theoretical introduction participants will take flow and pressure measurements on a small duct and fan system to develop the system curve, the fan curve, and the operating point for the system. The skills developed during this session can be used by the participants to determine the performance of the industrial ventilation systems at their individual facilities. A boxed lunch will be provided.

The Lunch and Learn will be held at 12303 Airport Way, Suite 200 in Broomfield, CO. Space is limited to 10 participants. Please contact Keith Robinson at or at 303-746-8904 to reserve your spot.

Please visit my main website at for more information about this Lunch and Learn or Keith D. Robinson, P.E.