Christoph Gmelin

Active Flow Control Concepts on a Highly Loaded Subsonic Compressor Cascade: Résumé of Experimental and Numerical Results

This paper presents experimental and numerical results for a highly loaded, low speed, linear compressor cascade with active flow control. Three active flow control concepts employing steady jets, pulsed jets, and zero mass flow jets (synthetic jets) are investigated at two different forcing locations: at the end walls and the blade suction side. Investigations are performed at the design incidence for jet-to-inlet velocity ratios of approximately 0.7 to 3.0 and two different Reynolds numbers. Detailed flow field data are collected using a five-hole pressure probe, pressure tabs on the blade surfaces, and time-resolved particle image velocimetry. Unsteady Reynolds-Averaged Navier-Stokes simulations are performed for a wide range of flow control parameters. The experimental and numerical results are used to understand the interaction between the jet and the passage flow. Variation of jet amplitude, forcing frequency and blowing angle of the different control concepts at both locations allows determination of beneficial control parameters and offers a comparison between similar control approaches. This paper combines the advantages of an expensive yet reliable experiment and a fast but limited numerical simulation. Excellent agreement in control effectiveness is found between experiment and simulation.

Investigations of Secondary Flow Suction in a High Speed Compressor Cascade

Numerical and experimental results for a high-speed compressor cascade with secondary flow suction are presented. Steady flow suction of low momentum fluid from the back flow region in the corner between end wall and vane is considered in order to diminish the corner separation. Investigations are performed at the design point with an inlet Mach number of 0.67 and a Reynolds number of 560,000 based on axial chord and inlet velocity. The steady Reynolds-Averaged Navier-Stokes simulations are evaluated against data from the accompanying experiment collected with pitot tubes and Conrad angle probes. Laminar separation bubbles on both suction and pressure surface are observed. Thus, transition from laminar to turbulent flow is respected in the simulations. The uncontrolled base flow case and various suction ratios (ratio of drawn to passage mass flow) are exploited. Additionally, the position of the slot is varied numerically. It is found that relocation of the slot slightly away from the suction surface improves the performance of the flow suction.Copyright

On the Efficiency of Secondary Flow Suction in a Compressor Cascade

An experimemtal investigation in a high speed compressor cascade has been carried out to show the effect of different types of secondary flow suction. In order to get deeper insight into the separated three dimensional flow topology and to determine appropriate suction positions, numerical simulations are performed additionally for the baseline cascade. To obtain the flow solution, an implicit, pressure based solver, elaN3D (by ISTA TU Berlin), is employed in steady RANS mode, whereby the Menter SST-k model is used for turbulence treatment. Both investigations are conducted at Mach number Ma = 0.67 and Reynolds number Re = 560.000. The aerodynamic design condition is used. The examined cascade consists of NACA65-K48 type vanes. The experiments include measurements with four different types of suction geometries plus reference measurements. Total pressure and flow angle measurements in the wake show the flow deflection, total pressure loss and the rise of the static pressure of the cascade. The best suction geometry follows the design of R.E. Peacock, designed for low Mach number cascades, with small changes. Using a maximum suction rate of 2% of the main flow the total loss coefficient was reduced by 23%. In this case the stage efficiency — calculated with a reference rotor — is increased by almost 1%. The vacuum pump energy consumption has been taken into account for this calculation. In another case the suction geometry has been chosen in a way that the suction slot is placed along the sidewall from suction side to pressure side following the wall streamlines. With an increased suction rate of 5% of the main flow, the vortex system in the passage is eliminated and the total loss coefficient is decreased to 0.055, which equals to a decrease of 37%. Taking into account that compressors in aero-engines provide bleed air for the plane’s air system, enormous efficiency increase is possible. For this the air bleed valves need to be redesigned.Copyright

Experimental and numerical results of active flow control on a highly loaded stator cascade

This article presents experimental and numerical results for a compressor cascade with active flow control. Steady and pulsed blowing has been used to control the secondary flow and separation characteristics of a highly loaded controlled diffusion airfoil. Investigations were performed at the design incidence for blowing ratios from approximately 0.7 to 3.0 (jet-to-inlet velocity) and a Reynolds number of 840 000 (based on axial chord and inlet velocity). Detailed flow field data were collected using a five-hole pressure probe, pressure taps on the blade surfaces, and time-resolved Particle Image Velocimetry. Unsteady Reynolds-averaged Navier–Stokes simulations were performed for a wide range of flow control parameters. The experimental and numerical results are used to understand the interaction between the jet and the passage flow. The benefit of the flow control on the cascade performance is weighted against the costs of the actuation by introducing an efficiency which takes the presence of the jets into account.

Active Flow Control Concepts on a Highly Loaded Subsonic Compressor Cascade: Résumé of Experimental and Numerical Results

The paper presents experimental and numerical results for a highly loaded, low speed, linear compressor cascade with active flow control. Three active flow control concepts by means of steady jets, pulsed jets, and zero mass flow jets (synthetic jets) are investigated at two different forcing locations, i.e. at the end walls and the blade suction side. Investigations are performed at the design incidence for jet-to-inlet velocity ratios from approximately 0.7 to 3.0 and two different Reynolds numbers. Detailed flow field data are collected using a five-hole pressure probe, pressure tabs on the blade surfaces, and time-resolved particle image velocimetry. Unsteady Reynolds-Averaged Navier-Stokes simulations are performed for a wide range of flow control parameters. The experimental and numerical results are used to understand the interaction between the jet and the passage flow. Variation of jet amplitude, forcing frequency, and blowing angle of the different control concepts at both locations allows determination of beneficial control parameters and offers a comparison between similar control approaches. The paper combines the advantages of an expensive but accurate experiment and a fast but limited numerical simulation.Copyright

Simulations of a Compressor Cascade with Steady Secondary Flow Suction

The paper presents numerical results for a high-speed compressor cascade with active flow control. Steady secondary flow suction is used to diminish the corner separation by removing low momentum fluid from the back flow area in the corner between side wall and vane. Investigations are performed at an inlet Mach number of 0.67 and a Reynolds number of 560,000 based on axial chord and inlet velocity. Steady Reynolds-Averaged Navier-Stokes simulations are performed for the uncontrolled base flow and suction ratios from approximately 0.5 to 2.0 (ratio of drawn to passage mass flow). The outcome of the calculations is evaluated against measurements from the accompanying experiments.

URANS Simulations of Active Flow Control on Highly Loaded Turbomachinery Blades

Active flow control is applied on highly loaded turbomachinery blades in order to delay separation, diminish secondary flow effects, and thus increase their efficiency. The impact is investigated separately in the context of two key configurations, i.e. a compressor cascade and an axial fan. Unsteady Reynolds-Averaged Navier-Stokes simulations are performed to determine beneficial flow control parameters. The results are backed up by autonomous experiments. The work concentrates on the numerical feasibility and the correct prediction of the flow control impact without claiming to present a complete analysis of the complex flow phenomena involved.

Unsteady RANS Simulations of a Highly Loaded Low Aspect Ratio Compressor Stator Cascade With Active Flow Control

A highly loaded compressor cascade is analyzed by means of time-resolved 3D RANS simulations. Due to the low aspect ratio of the cascade, strong three-dimensional effects emerge, such as large corner vortices and trailing edge separation at the midspan. The feasibility of the simulation using a commercial software and the applicability of controlling the separated regions using zero net mass flux synthetic jets is analyzed. The work includes two control concepts that are investigated separately. One aims to affect the secondary flow emerging from the sidewalls via actuation at the cascade casing walls. The other aims to reattach the separated flow to the blade suction side using an actuator on the blade. Beneficial flow control parameters characterizing a synthetic jet are determined for both locations by a systematic variation. Special attention is drawn to the global efficiency of the stator cascade by means of total pressure loss and pressure rise.Copyright

Numerical Investigations of Active Flow Control Using Synthetic Jets on a Highly Loaded Compressor Stator Cascade

Time-resolved Reynolds-Averaged Navier-Stokes simulations of a 3D stator compressor cascade are performed. At the design point of the airfoil under investigation, pronounced secondary flow effects are observed. Strong corner vortices emerge from the casing walls and the flow separates from the blade suction side towards the trailing edge. Transition from laminar to turbulent flow occurs within a laminar separation bubble. Using a commercial CFD software, the influence of the spatial resolution is investigated by means of a spanwise coarsening and refinement of the created mesh. Zero net mass flux synthetic jet actuation is used to control the separated regions. The work presents a variation of the temporal discretization and an analysis of the driving parameters of the actuation.Copyright

On the Performance of Boundary Layer Suction for Secondary Flow Control in a High Speed Compressor Cascade

A numerical study and experimental investigations have been performed in order to compare different boundary layer suction slot geometries and suction velocity ratios and rate their performance. The numerical study, also performed with secondary flow suction, has been done with a RANS solver using two different setups. In the first setup the suction slot itself is not calculated, but only present as an outflow region with constant outflow velocity no recirculation can appear. The second setup contains a fine resolved suction slot with representative height and width to clarify the influence of in-slot recirculation. The calculations are used as a basis for the comparison of suction slots used in high speed wind tunnel experiments. All investigations have been performed at a Mach number of Ma1 = 0.67 and a Reynolds number of Rec = 560.000. In the simple numerical simulation, the cascade total pressure loss is a function of the suction rate and is not affected by the suction slot size and therefore the suction velocity. In the experiments, a strong influence of the in-slot suction velocity to the efficiency is present. This can also be detected from the calculations of the cascade with attached suction slot. The authors state that for the boundary layer suction to increase the compressor performance effectively, the velocity from passage to slot must be positive throughout the orifice and recirculation from slot to passage must not appear.