Research at DLR Göttingen on bluff body aerodynamics, drag reduction by wake ventilation and active flow control

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Abstract

This paper outlines some experimental investigations on aerodynamics of bluff bodies which have been carried out at the former DLR Institute of Fluid Mechanics, now Institute of Aerodynamics and Flow Technology. Examples given here include (a) drag reduction by passive ventilation of the wake of the body, (b) reduction of the drag of automobiles by shape optimisation to eliminate or at least decrease trailing vortices, (c) optimisation of front and rear ends of high speed trains, (d) determination of the main sources of aerodynamic noise of high velocity trains which result from unsteady flow separations, (e) experiments on active flow control to delay flow separation.

Introduction

The flow around bluff bodies shows extremely interesting and technically important features. It is mainly characterised by large separation regions which in most cases are highly unsteady. Massive flow separations result in high drag forces on the bluff body. Unsteady vortex shedding into the wake flow can lead to severe buffeting of the body, especially if resonance effects occur. Also, unsteady flow separations are the main sources of aerodynamically generated noise often called “wind noise”.

Therefore, at DLR Göttingen, in two institutes, substantial investigations have been made and are still continuing to better understand the aerodynamics of bluff bodies. The DLR Institute of Aeroelasticity has extensive experience in the problems of flutter and buffeting of structures excited by unsteady separation, for example aeroplanes or large bridges. The DLR Institute of Aerodynamics and Flow Technology (former “DLR Institute of Fluid Mechanics”) is mainly involved in the following topics of bluff body aerodynamics:

  • •

    Understanding the flow structures of wake regions. Advanced remote sensing techniques like particle image velocimetry (PIV) and pressure sensitive paint (PSP) are employed in experiments on this subject.

  • •

    Reduction of wake size and suction pressure in the wake of bluff bodies, and reduction of the unsteadiness of wake flow. The objective is to decrease aerodynamic drag and fluctuating forces on the body. Developments of dynamic flow control methods and of new static flow control devices are the major issues of our research in this field.

  • •

    Investigation of sources of aerodynamic noise. Measurements of sound source intensity at wind tunnel models with significant local flow separation shed light on the characteristics of this important type of aerodynamic noise which is caused by the pressure fluctuations on the body surface induced by the unsteady wake flow. Full-scale tests on high-speed trains have confirmed and enhanced the findings from the wind-tunnel experiments.

This paper presents some typical examples of our work on the above topics.

Section snippets

Passive ventilation of the wake of bluff bodies

The drag of bluff bodies is determined mainly by the pressure drag caused by flow separation. The separation region, or wake, often extends over a larger cross-section than the cross-section of the body. This leads to a big pressure difference between front and rear surfaces. Additionally, the shear layers of the separated flow tend to be unstable, and concentration of vorticity results in huge vortices in the wake. Because of this unsteadiness the drag forces are also unsteady with respect to

Car aerodynamics

Already during the 1930s substantial work on automobile configurations with low drag has been performed at the predecessor of DLR Göttingen, the Aerodynamische Versuchsanstalt (AVA). As an example, Fig. 8 shows the so-called “AVA-Schlör car” in a road test.

Results of these road tests and of wind-tunnel measurements with scale models and full-scale vehicles are given in Fig. 9, from Ref. [5, pp. 23, 24]. Very low drag coefficients were already obtained with the Schlör configuration, probably at

Railway aerodynamics and acoustics

A special contribution of the DLR to the development of the Intercity-Express trains was the design of the front and rear ends of the first Intercity-Express generation of the German railway. Because these trains are running in both directions not exchanging front and rear, a compromise had to be found between the designs for forward- and backward-oriented flow. Also, the problem of lift was considered carefully because a high-speed train has the problem of unstable motion on the rails under

Lifting surfaces at high angles of incidence

At high angles of incidence, even streamlined bodies tend to show massive flow separation. First, the appearance of stream-wise vortices indicates separation on the upper surface of the body. At higher angles of attack these vortices typically become unsteady and a periodic vortex shedding is the final result. These effects are observed as well in the case of cylindrical bodies (rockets) as in the case of delta wing or other aircraft configurations. Wings of high-aspect ratios show leading-edge

Conclusions

We have discussed some investigations on aerodynamics of bluff bodies at the DLR Institute of Aerodynamics and Flow Technology including drag reduction by passive ventilation of the wake of the body, reduction of the drag of automobiles by shape optimisation, investigation of the main sources of aerodynamic noise of high-velocity trains which result from unsteady flow separations, and experiments on active flow control to delay flow separation from airfoils at high angles of incidence. Special

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