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A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.

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The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25 · 10 5 to 3.75 · 10 5 . Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δc l ≈ 0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic stall vortex were fully suppressed. This led to a significant reduction in lift hysteresis and form drag while simultaneously mitigating moment coefficient excursions.airfoil pitching frequency, Hz h = control slot height (1.2 mm) k = reduced pitching frequency; πfc∕U ∞ M = Mach number N = number of samples Re = Reynolds number; U ∞ c∕ν s = airfoil span (610 mm) u = streamwise velocity component (wind-tunnel frame of reference), m∕s U e = local boundary-layer edge velocity, m∕s U j = blowing jet velocity, m∕s U ∞ = wind-tunnel speed, m∕s _ V = volumetric blowing flow rate, m 3 ∕s x, y = chordwise and chord-normal positions (airfoil frame of reference), m x 0 , y 0 = streamwise and normal positions (wind-tunnel frame of reference), m y m = distance of the point of maximum jet velocity from the wall, m α = angle of attack, deg α s = static stall angle, deg Δc l = relative change in lift coefficient produced by control η = angle of the control slots relative to the airfoil surface (20 deg) ν = kinematic viscosity, m 2 ∕s σ c l = standard deviation of the lift coefficient σ c m = standard deviation of the moment coefficient ϕ= phase angle of the sinusoidal pitching motion, deg ω = airfoil pitching angular frequency; 2πf ω z = nondimensional vorticity; ∂v∕∂x − ∂u∕∂y · c∕U ∞

In civil engineering, many structures are made of reinforced concrete. Most degradation processes relevant to this material, e.g., corrosion, are related to an increased level of material moisture. Therefore, moisture monitoring in reinforced concrete is regarded as a crucial method for structural health monitoring. In this study, passive radio frequency identification (RFID)-based sensors are embedded into the concrete. They are well suited for long-term operation over decades and are well protected against harsh environmental conditions. The energy supply and the data transfer of the humidity sensors are provided by RFID. The sensor casing materials are optimised to withstand the high alkaline environment in concrete, having pH values of more than 12. Membrane materials are also investigated to identify materials capable of enabling water vapour transport from the porous cement matrix to the embedded humidity sensor. By measuring the corresponding relative humidity with embedded passive RFID-based sensors, the cement hydration is monitored for 170 days. Moreover, long-term moisture monitoring is performed for more than 1000 days. The experiments show that embedded passive RFID-based sensors are highly suitable for long-term structural health monitoring in civil engineering. damage diagnosis and assessment. Nevertheless, it is difficult to obtain the required information for distinct damage mechanisms, especially if the damage emanates from inside the concrete structure. For example, chloride-induced corrosion occurs on the surface of reinforcing bars in steel-reinforced concrete. The initiation of this process cannot be detected by traditional visual inspections [5]. Rather, it is typically only after years or even decades of degradation that obvious damage can be observed on the surface of a structural component, i.e., cracks, rust stains, delamination of the concrete cover, etc. At this point, massive damage is already present, and the corresponding repair work is expensive. Early detection of the onset of corrosion reduces costs, ensures building safety, and may increase the lifetime of the structure [6][7][8]. Thus, proper monitoring of crucial infrastructure like bridges, pavements, and foundations will reduce the overall costs of our infrastructure significantly.As a complement to visual inspections, sensors are capable of acquiring further information. Such sensors are particularly useful for collecting information from locations that are difficult to access, such as foundations, bridge bearings, and the bottom side of bridge decks, as well as locations where access is prohibited or dangerous, such as in nuclear waste containments. Appropriate sensors measure several quantities, which give a deep insight into the structural condition. To advance the state-of-the-art in this crucial field the "Bundesanstalt für Materialforschung und -prüfung" (BAM, Federal Institute for Materials Research and Testing) develops sensors designed for civil engineering structures. In the current study, the focus is on the det...

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