Frank Haucke

Large scale separation flow control experiments within the German Flow Control Network

This is an overview on flow control experiments for flow separation control conducted in the DNW-NWB atmospheric low-speed wind tunnel performed within the German Flow Control Network. Emphasis is given on the experimental setup using the DLR F15 wall-to-wall two-dimensional high-lift model. Examples of successful flow control for enhancement of lift are given for leading edge boundary layer control and flap separation control, both by means of pulsed jet actuation.

High-Lift Performance Investigation of a Two-Element Configuration with a Two-Stage Actuator System

cL = lift coefficient cp = pressure coefficient cref = reference chord length, m cμ = momentum coefficient, _ m · ujet;theo ∕ q∞ · Aref F , Sr = normalized frequency, Strouhal number, f · cchar ∕ uchar f = frequency, Hz _ m = total mass flow rate, kg∕s _ md = driving stage mass flow rate, kg∕s _ mo = outlet stage mass flow rate, kg∕s pstat = static pressure, bar p0 = total pressure, bar R d∕o = ratio of mass flow rate through driving and outlet stage, _ md∕ _ mo u, v, w = velocity in x, y, and z directions, m∕s ujet;theo = velocity of air jet calculated frommass flow rate,m∕s u∞ = freestream velocity, m∕s jVj = absolute velocity from u, v, and w components, m∕s x = streamwise direction, mm y = direction normal to tunnel floor, mm z = (spanwise) direction normal to tunnel side wall, mm α = angle of attack, deg δf = incidence angle of flap, deg

Robust Closed-Loop Control on a 2D-High-Lift-Device

This work shows the application of robust closed-loop strategies on a 2D high-lift configuration. The aim is to control the amount of lift achieved, instead of maximising that amount as it is usually done with open-loop set-ups. A way to model a 2D high-lift-device will be outlined, based on linear black-box-models of the lift as a function of the actuation amplitude. This model will be examined with respect to needed complexity and sensitivity to experimental parameters. In an experimental implementation the disturbance rejection and tracking capability of a synthesized closed-loop-controller will be shown and its possible use in aviation pointed out.

Enhanced Convective Heat Transfer due to Dynamically Forced Impingement Jet Arrays

Within the framework of the German collaborative research centre “SFB1029”, dynamically forced impingement cooling is investigated experimentally. This will provide a contribution to compensate the critical effects of a turbine inlet temperature increase induced by innovative combustion concepts. The present study describes experimental investigations regarding dynamically forced heat transfer between a flat hot surface and an array of up to 7 by 7 circular impingement jets. Fast switching solenoid valves are used to periodically pulse each cooling jet separately by changing frequency, duty cycle and phasing. Depending on the excitation parameters, strong ring vortices can be generated around each single jet. Thereby the maximum velocity within the core zone of each single jet can be significantly increased. Simultaneously, the vorticity is increased, which induces higher local and temporal wall shear stress after impinging on the wall and thus enhanced forced convective heat transfer as well. Considering a multi nozzle arrangement, the vortex structures of each impingement jet will interfere with the adjacent ones, which strongly influences cooling effectiveness.Copyright

Combined Active Separation Control on the Leading Edge and on the Trailing Edge Flap of a Slatless High-Lift Configuration

This paper describes experimental investigations into active flow control applied at the wing leading edge and at the trailing edge flap of a slatless high-lift wing configuration. The experiments were conducted on an airfoil, which is used extensively by different German and European research programs. The experimental results presented in this work demonstrate successfully the effectiveness of multi-locational active flow control to increase \(c_l\), \(c_{l,max}\) and \(\varDelta \alpha _{max}\) in combined mode.

Experimental Study on the Alteration of Cooling Effectivity Through Excitation-Frequency Variation Within an Impingement Jet Array with Side-Wall Induced Crossflow

The influence of in-phase variation of the excitation frequency of a 7 by 7 impinging jet array between \(f=0\) and 1000 Hz on the cooling effectivity is investigated experimentally. Liquid crystal thermography is employed to measure a 2-dimensional wall-temperature distribution, which is used to calculate the local Nusselt numbers and evaluate the global and local heat transfer. The cooling effectivity of the dynamic approach is determined by comparison with corresponding steady blowing conditions. The results show that the use of a specific excitation frequency allows a global cooling effectivity increase of more than 50%.

Map Estimation for Impingement Cooling with a Fast Extremum Seeking Algorithm

In many actively controlled processes, such as active flow or combustion control, a set of actuation parameters has to be specified, ranging from actuation frequency to pulse width to geometrical parameters such as actuator spacing. As a specific example, impingement cooling is considered here. Finding the optimal parameters for impingement cooling with steady-state measurements is a time consuming process because of the necessary time to reach thermal equilibrium. This work presents an algorithm for fast extremum seeking to reduce the amount of time needed. It is inspired by an Extremum Seeking Controller, which is a simple but powerful feedback control technique. The first results using this concept are promising, as the magnitude of the optimal pulse frequency for the cooling efficiency of pulsed impingement jets could be found with sufficient precision in a short period of time. The main advantages of this concept are the simple execution on a test rig, its versatility, and the fact that almost no information about the investigated system is necessary.

Experimental Investigation of a Dynamically Forced Impinging Jet Array

Dynamic forced impingement cooling is investigated experimentally. Due to the pulsation of impinging jets, strong ring vortices can be generated. Thereby, pulse parameters, e.g. frequency, amplitude, duty cycle and phase shift as well as geometrical parameters e.g. impingement distance, jet nozzle distance or jet nozzle arrangement, play an important role on maximizing the heat transfer on hot surfaces. In the present paper the local convective heat transfer on an electrically heated flat plate is being investigated, while under the influence of a 7 by 7 inline impinging jet array. The cooling efficiency of pulsed actuation with side wall induced crossflow is determined by comparing local Nusselt numbers to the steady blowing case.