Experiments are conducted to investigate the ability of variable-pressure pulsed-blowing actuation to maintain a constant lift force on a low-aspect-ratio semicircular wing in a longitudinally gusting flow. Dynamic models of the lift response to actuation and the lift response to longitudinal gusting are obtained through modern system identification methods. Robust closed-loop controllers are synthesized using a mixed-sensitivity loop-shaping approach. An additional feedforward disturbance compensator is designed based on a model of the unsteady aerodynamics. The controllers show suppression of lift fluctuations at low gust frequencies, f < 0:8 Hz (reduced frequency, k < 0:09). At higher frequencies, the control performance degrades due to limitations related to the time for a disturbance, created by the actuators, to convect over the wing and establish the flowfield that leads to enhanced lift on the wing.

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The ability to control lift in unsteady flows using active flow control is examined experimentally with a three-dimensional, low-aspect-ratio wing and pulsedblowing actuators as the test article. An unsteady flow wind tunnel is used to generate step-like and harmonic oscillations in flow speed and the corresponding fluctuating lift force on the wing. A 'black box' model of the wing response to actuation is obtained using conventional system identification techniques. A robust H ∞ controller is designed with a mixed sensitivity loop-shaping technique, whose objective was to maintain a constant lift in the unsteady flow. The controller is shown to be capable of significant reductions in lift fluctuations given step, harmonic and random input disturbance conditions.

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This contribution presents a new optimization procedure for the design of the appendix gap in Stirling engines. This is of special interest, because the thermal losses associated with the appendix gap, an annular gap around the displacer in the cylinder system, may amount to as much as 10% of the heat input to a Stirling engine. Furthermore, recent experimental investigations and an extensive literature review revealed that the accuracies of the existing models for these losses and the derived design rules for the appendix gap are insufficient. The optimization procedure presented here is based on a recently developed and published, more detailed analytical model. To generally analyze any optimization potential, this model is first subject to numerical optimization. The choice of the correct gap width implies a major optimization potential, but there are additional options (e.g., choosing a conical design with an expansion towards the open end). Thus, the loss can be reduced by more than 10%. Furthermore, the gap loss can be reduced by a modification of the seal design, reducing the volumetric displacement and the almost isothermal buffer volume at the bottom end of the gap. If the gap width is minimized in the bottom section of the gap, a larger optimum width is obtained for the remaining length of the gap, and no conicity is needed. Finally, an analytical, easily applicable, closed-form approximate solution for the optimum gap width and the corresponding overall gap loss has been derived and validated. Nomenclature A = area, m 2 a = thermal diffusivity, m 2 ∕s b = complex constant c D = specific heat capacity of the displacer wall, J∕kg · K c h = conicity c p = specific isobaric heat capacity, J∕kg · K d = diameter, m f = factor for the evaluation of the optimum Péclet number H = enthalpy, J h = appendix gap width, m J = imaginary part i = imaginary unit (i −1 p ) J = function (velocity ratio) k = complex constant l = length, m m = mass, kg n = rotational speed, s −1 Pe ω = kinetic Péclet number (Pe ω 4h 2 ω∕a) Pr = Prandtl number (Pr ν∕a) p = pressure, Pa Q = heat, J R = specific gas constant, J∕kg · K R = real part Re ω = Valensi number (Re ω 4h 2 ω∕ν) r h = bottom gap width ratio r p = dimensionless pressure amplitude r x = dimensionless stroke amplitude T = Temperature, K u = flow velocity in x-direction, m∕s V C = swept volume of cylinder, m 3 x = axial coordinate, m x = displacer stroke amplitude, m Y = temperature gradient, K∕m Y 0 = overall temperature gradient, K∕m y = coordinate transverse to the gap, m Γ = velocity ratio (Γ û m ∕xω) δ = relative deviation Θ = dimensionless temperature θ p = phase angle of pressure relative to displacer velocity θ u = phase angle of mean flow velocity relative to displacer velocity Λ = complex factor (Poiseuille flow) λ = thermal conductivity, W∕m · K ν = kinematic viscosity, m 2 ∕s Ξ = complex constant ρ = density, kg∕m 3 Σ = overall loss, W φ = crank angle ω = angular velocity of crank shaft, s −1 ω 2πn Subscripts a = appendix gap ad = adiabatic bot = bottom end of ...

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