Deterministic Disturbance Generator

Project Motivation

Increasing air travel has lead to a growing public concern about aircraft and engine noise. In 1990 Congress passed the Airport Noise and Capacity Act which demanded noise reductions in all aircraft by 2000 [1]. Much of the noise produced by aircrafts results from the unsteady flow fluctuations around the aircraft, and its interaction with lifting surfaces, wings, tails, and support structures [2]. These problems are not limited to aircraft, but affect all vehicles operating in a fluid including boats, submarines, and cars.
The key to solving any aeroacoustics problem is determining the aerodynamic response function of the system being investigated. Traditionally uniform turbulence fields, created by placing a grid of fixed bars ahead of a model, have been used for experimental aeroacoustics studies. These static grids could not produce turbulence large enough to be comparable to real vehicular flows[3]. More recently, improvements in grid technology led to the development of active turbulence generation grids (ATGs) capable of producing large scale turbulence. ATGs allow some researchers to learn about aerodynamic response functions by comparing second order statistical quantities, such as surface pressure spectrum on a model, with the spectra in the undisturbed turbulence field[4]. This analysis has several shortcomings. Only the square of the response function can be found, resulting in dynamic range limitations and amplifying the effect of measurement noise in the results[5]. Additionally, all time domain information is lost, so casual relations are lost or hidden. In order to better study vehicular noise a method must be created to allow for the retention of time domain information.

Project Goals

We are developing and studying an active deterministic disturbance generator (DDG), which allows the retention of time domain information in aeroacoustics studies. The device shown in Figure 1 can produce disturbances containing only a narrow band of frequency or amplitude scale patterns, which can be periodically repeated in a controllable wind tunnel environment. The generator can for example produce a continuous unsteady flow pattern where the flow fluctuations are of the same amplitude and frequency. Two different modes of operation can be viewed in Movie1 (.mpg 3.9M) and Movie2 (.mpg 4.5M). The effect of a vehicle model could be seen in the distortion of these disturbances in the time domain. Ensemble averages of the disturbance passing over the vehicle will allow the retention of time domain information and will average out the effects of measurement inaccuracies and noise. First order statistics can also be used which provide more physical insight than the second order statistics used with isotropic turbulence measurements.
Figure 1. DDG in test frame

The DDG was constructed and tested during 2002 and 2003. The generator contains ten airfoil shaped actuators that are manipulated by a set of high torque stepper motors. Initial tests conducted in the Virginia Tech Stability Wind Tunnel showed that the generator is functional and can provide large repeatable disturbance patterns. While this is encouraging, work still remains before the project is considered a success. Once the reliability and uniformity of the generator are improved, it can be used to study the flow around a vehicle model.
We will study the behavior of turbulence passing through a set of parallel thin flat plates placed across the flow in the wind tunnel, seen in Figure 2. This configuration is also being studied by JV Larssen with subject to an isotropic turbulence field created by an ATG. This configuration can be viewed as a simplified version of engine supports or of the stators found in jet engines and submarine propeller ducts. The configuration is also simple enough to be accurately modeled and simulated using Rapid Distortion Theory or Large Eddy Simulation.
Figure 2. Flat plate cascade

Project Status

  1. The DDG has been designed, built, and electromechanically debugged. See DDG Construction (.pdf, 1.1M).
  2. The DDG has been used in the Stability wind tunnel and the created flow field has been studied. See Initial wind tunnel testing of DDG (.pdf 1.5M).
  3. Ideal flow predictions have been made of the flow produced by the DDG.

Future Work

  1. Complete modifications to improve grid reliability and performance.
  2. Verify tunnel entry and grid/tunnel calibration procedure, and prove repeatability.
  3. Document flow field at the entrance of the flat plate cascade.
  4. Measure flow response of deterministic disturbances over a flat plate cascade.

References

  1. Airport Noise and Capacity Act (1990) Pub.L. 101-508
  2. Scharpf, D. F. Mueller, T J., “Experimental investigation of the sources of propeller noise due to the ingestion of turbulence at low speeds Experiments in Fluids”, v 18 n 4 Feb 1995. p 277-287.
  3. Patterson, R. W. and Amiet R. K., 1976, “Acoustic radiation and surface pressure characteristics of an airfoil due to incident turbulence”, NASA CR2733
  4. Larssen, J. V. and Devenport, W. J., 2002, “The Generation of High Reynolds Number Homogeneous Turbulence”, AIAA-2002-2861, 32nd AIAA Fluid Dynamics Conference and Exhibit, 24-26 June 2002, St. Louis, Missouri.
  5. Mish, P, 2003, “An Experimental Investigation of Unsteady Surface Pressure on Single and Multiple Airfoils”, Ph.D. Dissertation, Virginia Tech.

Last Updated: 07/31/2004