Hot Wind Research Articles

The growth and entrainment of cold gas in a hot wind

ABSTRACT Both absorption and emission-line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km s−1. This observational fact poses a severe challenge to our theoretical models of galaxy evolution since most feedback mechanisms (e.g. supernovae feedback) accelerate hot gas, and the time-scale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, ‘warm’ gas is sufficiently large to contribute new comoving cold gas, which can significantly exceed the original cold gas mass. This happens whenever, tcool, mix/tcc < 1, where tcool, mix is the cooling time of the mixed warm gas and tcc is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling ‘focuses’ stripped material on to the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behaviour.

Gas expulsion versus gas retention in young stellar clusters–II. Effects of cooling and mass segregation

Gas expulsion or gas retention is a central issue in most of the models for multiple stellar populations and light element anti-correlations in globular clusters. The success of the residual matter expulsion or its retention within young stellar clusters has also a fundamental importance in order to understand how star formation proceeds in present-day and ancient star-forming galaxies and if proto-globular clusters with multiple stellar populations are formed in the present epoch. It is usually suggested that either the residual gas is rapidly ejected from star-forming clouds by stellar winds and supernova explosions, or that the enrichment of the residual gas and the formation of the second stellar generation occur so rapidly, that the negative stellar feedback is not significant. Here we continue our study of the early development of star clusters in the extreme environments and discuss the restrictions that strong radiative cooling and stellar mass segregation provide on the gas expulsion from dense star-forming clouds. A large range of physical initial conditions in star-forming clouds which include the star-forming cloud mass, compactness, gas metallicity, star formation efficiency and effects of massive stars segregation are discussed. It is shown that in sufficiently massive and compact clusters hot shocked winds around individual massive stars may cool before merging with their neighbors. This dramatically reduces the negative stellar feedback, prevents the development of the global star cluster wind and expulsion of the residual and the processed matter into the ambient interstellar medium. The critical lines which separate the gas expulsion and the gas retention regimes are obtained.

On the X-ray temperature of hot gas in diffuse nebulae

X-ray emitting diffuse nebulae around hot stars are observed to have soft-band temperatures in the narrow range [1-3]$\times10^{6}$ K, independent of the stellar wind parameters and the evolutionary stage of the central star. We discuss the origin of this X-ray temperature for planetary nebulae (PNe), Wolf-Rayet nebulae (WR) and interstellar wind bubbles around hot young stars in our Galaxy and the Magellanic Clouds. We calculate the differential emission measure (DEM) distributions as a function of temperature from previously published simulations and combine these with the X-ray emission coefficient for the 0.3-2.0 keV band to estimate the X-ray temperatures. We find that all simulated nebulae have DEM distributions with steep negative slopes, which is due to turbulent mixing at the interface between the hot shocked stellar wind and the warm photoionised gas. Sharply peaked emission coefficients act as temperature filters and emphasize the contribution of gas with temperatures close to the peak position, which coincides with the observed X-ray temperatures for the chemical abundance sets we consider. Higher metallicity nebulae have lower temperature and higher luminosity X-ray emission. We show that the second temperature component found from spectral fitting to X-ray observations of WR nebulae is due to a significant contribution from the hot shocked stellar wind, while the lower temperature principal component is dominated by nebular gas. We suggest that turbulent mixing layers are the origin of the soft X-ray emission in the majority of diffuse nebulae.

Biological rhythmicity of the plasma antioxidant defence in lambs following supplementation of micronutrients or providing shelter in temperature-controlled microenvironment in summer

Antioxidant defence is one of the important biological adaptations to thermal stress in animals. Micronutrients play crucial roles in the bioactivity of several antioxidant systems. Shelter management by providing a thermo-comfort micro-environment to animals may also combat the heat stress during summer. Hence, the aim of the present study was to assess the biological ability of Malpura lambs to counter thermal stress following supplementation of micronutrients and providing shelter in a temperature-controlled climatic chamber during the hot day-hours in summer. Twenty-one lambs of 3 - 5 months age were equally divided in to three group viz. Group1: control, Group2: micronutrient- supplemented and Group3: housing in thermo-neutral climatic chamber. Lambs in group1 and 2 were maintained in a shed having free access to hot wind while the lambs in group 3 were kept inside a climatic chamber (temperature and relative humidity about 30 °C and 40%, respectively) between 10 am to 4:30 pm. All the lambs were provided identical diet consisting of 2.5 kg concentrate and ad libitum quantities of Cenchrus ciliaris hay and clean water. Group3 lambs were additionally supplemented with 0.150 g micronutrient pellet per animal per day for a period of 30 days. Blood samples were collected at 0, 15, 30 days and plasma antioxidants activity were estimated. The catalase activity in the micronutrient-pellet supplemented group was significantly (p < 0.05) higher than the control both at day15 and day30. The superoxide dismutase activity was increased significantly (p < 0.05) at day30 than the day0 in both the micronutrient-supplemented and climatic chamber groups. In conclusion, both the micronutrient supplementation and housing in a thermo-comfort environment enhance the antioxidant defence and biological adaptation in lambs during thermal stress in summer.

Tempering of water storage tank temperature in hot climates regions using earth water heat exchanger

In hot climate regions, outdoor domestic water tank is directly exposed to high solar intensity in addition to hot summer wind stream. The storage water temperature increases above 50 °C. In order to overcome this problem, time dependent analysis of cylindrical domestic storage water tank that exposed to high solar radiation is performed. The analysis includes energy balance for the water tank as well as to an earth water heat exchanger used to reduce the water temperature in summer months. The governing equations are solved numerically using fourth order Runge-Kutta method. It is found that using earth water heat exchanger reduces the water temperature by about 16 °C. A comparison of the numerical results with experimental data showed a good agreement.

H i Kinematics along the Minor Axis of M82

Abstract M82 is one of the best-studied starburst galaxies in the local universe, and is consequently a benchmark for studying star formation feedback at both low and high redshift. We present new VLA H i observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of H i up to 10 kpc along the minor axis toward the south and beyond 5 kpc to the north. A surprising aspect of these observations is that the line-of-sight H i velocity decreases substantially from about 120 to from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the H i gas cooling from the hot wind. We demonstrate that the velocity decrease is substantially greater than the deceleration expected from gravitational forces alone. If the H i consists of a continuous population of cold clouds, some additional drag force must be present, and the magnitude of the drag force places a joint constraint on the ratio of the ambient medium to the typical cloud size and density. We also show that the H i kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the H i over the ∼1 kpc extent of the starburst region. Regardless of the launch mechanism for the H i gas, the observed velocity decrease along the minor axis is sufficiently great that the H i may not escape the halo of M82. The inferred H i outflow rate at 10 kpc off the midplane is much less than 1 yr−1.

An experimental investigation of cooling characteristics at a vane end-wall with a locally enhanced hole-layout

This paper presents an experimental investigation on the effect of film hole-layout at a turbine vane end-wall on the cooling performances, including the distributions of overall cooling effectiveness, uniformity of temperature field and highest temperature drop, using a hot wind tunnel and an Infrared Thermal Imaging System (ITIS). Two specimens were manufactured and used. One is named as “uniform hole-layout (UHL)”, where ten rows of film holes were uniformly arranged along the axial direction. However, UHL results in a non-uniform temperature distribution over the entire end-wall surface, and the low film coverage and bad reliability in the regions near leading edge (LE) and pressure side (PS) of the vane thereby. To improve the cooling performance in these regions, a new layout named as “locally enhanced hole-layout (LEHL)” was proposed. The difference from the previous experiments conducted under adiabatic assumption is that, the conjugate heat transfer is considered, and the conjugate-temperature distribution is used as the basis of design of LEHL. The comparisons of the cooling performances of two specimens at various mass flow ratios (MFRs) of coolant-to-mainstream revealed that: (1) from MFR = 0.5% to 1.5%, using LEHL, the area-averaged overall cooling effectiveness over the entire end-wall, in LE region and in the corner of PS can be increased by at least 32.3%, 34.2% and 26.2%, respectively; and (2) the temperature differences from PS to suction side (SS) and the temperature at the entire end-wall can be significantly reduced by LEHL. Therefore, the design strategy of new hole-layout using the conjugate-temperature is conducive to obtain the simultaneous improvement of film coverage and working life.

Dusty Cloud Acceleration by Radiation Pressure in Rapidly Star-forming Galaxies

Abstract We perform two-dimensional and three-dimensional radiation hydrodynamic simulations to study cold clouds accelerated by radiation pressure on dust in the environment of rapidly star-forming galaxies dominated by infrared flux. We utilize the reduced speed of light approximation to solve the frequency-averaged, time-dependent radiative transfer equation. We find that radiation pressure is capable of accelerating the clouds to hundreds of kilometers per second while remaining dense and cold, consistent with observations. We compare these results to simulations where acceleration is provided by entrainment in a hot wind, where the momentum injection of the hot flow is comparable to the momentum in the radiation field. We find that the survival time of the cloud accelerated by the radiation field is significantly longer than that of a cloud entrained in a hot outflow. We show that the dynamics of the irradiated cloud depends on the initial optical depth, temperature of the cloud, and intensity of the flux. Additionally, gas pressure from the background may limit cloud acceleration if the density ratio between the cloud and background is . In general, a 10 pc-scale optically thin cloud forms a pancake structure elongated perpendicular to the direction of motion, while optically thick clouds form a filamentary structure elongated parallel to the direction of motion. The details of accelerated cloud morphology and geometry can also be affected by other factors, such as the cloud lengthscale, reduced speed of light approximation, spatial resolution, initial cloud structure, and dimensionality of the run, but these have relatively little affect on the cloud velocity or survival time.

Numerical Simulations of Multiphase Winds and Fountains from Star-forming Galactic Disks. I. Solar Neighborhood TIGRESS Model

Abstract Gas blown away from galactic disks by supernova (SN) feedback plays a key role in galaxy evolution. We investigate outflows utilizing the solar neighborhood model of our high-resolution, local galactic disk simulation suite, TIGRESS. In our numerical implementation, star formation and SN feedback are self-consistently treated and well resolved in the multiphase, turbulent, magnetized interstellar medium. Bursts of star formation produce spatially and temporally correlated SNe that drive strong outflows, consisting of hot ( ) winds and warm ( ) fountains. The hot gas at distance from the midplane has mass and energy fluxes nearly constant with d. The hot flow escapes our local Cartesian box barely affected by gravity, and is expected to accelerate up to terminal velocity of . The mean mass and energy loading factors of the hot wind are 0.1 and 0.02, respectively. For warm gas, the mean outward mass flux through is comparable to the mean star formation rate, but only a small fraction of this gas is at velocity . Thus, the warm outflows eventually fall back as inflows. The warm fountain flows are created by expanding hot superbubbles at at larger d neither ram pressure acceleration nor cooling transfers significant momentum or energy flux from the hot wind to the warm outflow. The velocity distribution at launching near is a better representation of warm outflows than a single mass loading factor, potentially enabling development of subgrid models for warm galactic winds in arbitrary large-scale galactic potentials.

Implications of the Large O vi Columns around Low-redshift L∗ Galaxies

Abstract Observations reveal massive amounts of O vi around star-forming L * galaxies, with covering fractions of near unity extending to the host halo’s virial radius. This O vi absorption is typically kinematically centered upon photoionized gas, with line widths that are suprathermal and kinematically offset from the galaxy. We discuss various scenarios and whether they could result in the observed phenomenology (cooling gas flows, boundary layers, shocks, virialized gas). If collisionally ionized, as we argue is most probable, the O vi observations require that the circumgalactic medium (CGM) of L * galaxies holds nearly all of the associated baryons within a virial radius ( ) and hosts massive flows of cooling gas with , which must be largely prevented from accreting onto the host galaxy. Cooling and feedback energetics considerations require cm−3 K for the warm and hot halo gases. We argue that virialized gas, boundary layers, hot winds, and shocks are unlikely to directly account for the bulk of the O vi. Furthermore, we show that there is a robust constraint on the number density of many of the photoionized absorption systems that yields upper bounds in the range cm−3, suggesting that the dominant pressure in some photoionized clouds is nonthermal. This constraint is in accordance with the low densities inferred from more complex photoionization modeling. The large amount of cooling gas that is inferred could re-form these clouds in a fraction of the halo dynamical time, and it requires much of the feedback energy available from supernovae to be dissipated in the CGM.

Experimental study of flow and heat transfer in rotary air preheaters with honeycomb ceramics and metal corrugated plates

The objective of this paper is to compare flow and heat transfer performance of rotary air preheaters consisting of honeycomb ceramics and metal corrugated plates. The experimental study was carried out by a bi-directional switching hot wind tunnel filled with single heat transfer matrix. The heat transfer and resistance empirical correlations, as well, the relationships between effectiveness and number of transfer unit were deduced from measured data. Under the same conditions, higher outlet air temperature and heat transfer effectiveness, lower pressure drop occurred in the metal heat transfer elements. The effects of switching time and flue gas velocity on flow and heat transfer performance of rotary air preheaters were analyzed. The experimental approach of bi-directional switching hot wind tunnel filled with single heat transfer matrix could be served as a performance analysis method for rotary air preheaters with different materials and passage shapes.

Feeding the fire: Tracing the mass-loading of 107 K galactic outflows with O vi absorption.

Galactic outflows regulate the amount of gas galaxies convert into stars. However, it is difficult to measure the mass outflows remove because they span a large range of temperatures and phases. Here, we study the rest-frame ultraviolet spectrum of a lensed galaxy at z ~ 2.9 with prominent interstellar absorption lines from O i, tracing neutral gas, up to O vi, tracing transitional phase gas. The O vi profile mimics weak low-ionization profiles at low velocities, and strong saturated profiles at high velocities. These trends indicate that O vi gas is co-spatial with the low-ionization gas. Further, at velocities blueward of -200 km s-1 the column density of the low-ionization outflow rapidly drops while the O vi column density rises, suggesting that O vi is created as the low-ionization gas is destroyed. Photoionization models do not reproduce the observed O vi, but adequately match the low-ionization gas, indicating that the phases have different formation mechanisms. Photoionized outflows are more massive than O vi outflows for most of the observed velocities, although the O vi mass outflow rate exceeds the photoionized outflow at velocities above the galaxy's escape velocity. Therefore, most gas capable of escaping the galaxy is in a hot outflow phase. We suggest that the O vi absorption is a temporary by-product of conduction transferring mass from the photoionized phase to an unobserved hot wind, and discuss how this mass-loading impacts the observed circum-galactic medium.

Agroclimatic zoning of the northern grain-producing territory of Kazakhstan

Introduction. The existing agroclimatic handbooks in Kazakhstan are outdated in informational and technological relation. Therefore it is necessary to conduct reassessment of agroclimatic resources.&#x0D; Purpose. The agroclimatic zoning of the territory of northern Kazakhstan in terms of heat supply and moisture content.&#x0D; Methods. Data of meteorological stations over 1981-2014 were used. Methods of statistic and climatologic processing of data were applied. Agroclimatic maps were developed using the ArcGIS 10.1 software.&#x0D; Results. Analysis of spatial distribution of values of moisture coefficient K and sums of active air temperatures above 10оC provided an opportunity to single out 6 agroclimatic zones of plain territory of Kazakhstan. Zones from 3 to 6 are subdivided into two types in terms of thermal conditions. There are 3 agroclimatic zones singled out within the territory of North Kazakhstan Region, 4 zones – in Kostanai region, 4 zones – in Akmola region, 2 zones – in Pavlodar region. Distribution of drought characteristics, hot dry winds and climatic terms for beginning of spring field works within Akmola region's agroclimatic zones was given as an example for systematization of spatial distribution of respective values.&#x0D; The first zone with moderately humid and moderately warm climate has severe droughts with recurrence of 2%, hot winds of moderate intensity are observed during 2-3 days, climatic terms for beginning of spring field works fall on 15th-19th of May. The fourth zone with moderately dry and warm climate has severe droughts with recurrence of 30-35%, hot winds of moderate intensity are observed during 20-25 days, climatic terms for beginning of spring field works fall on 5th-7th of May.&#x0D; Conclusions. There are 3 agroclimatic zones singled out within the territory of North Kazakhstan Region, 4 zones – in Kostanai region, 4 zones – in Akmola region, 2 zones – in Pavlodar region.

Evolution of broad-band SED during outburst rise in NS X-ray Nova Aql X-1

The observed evolution of the broad-band spectral energy distribution (SED) in NS X-ray Nova Aql X-1 during the rise phase of a bright FRED-type outburst in 2013 can be understood in the framework of thermal emission from unstationary accretion disc with temperature radial distribution transforming from a single-temperature blackbody emitting ring into the multi-colour irradiated accretion disc. SED evolution during the hard to soft X-ray state transition looks curious, as it can not be repro- duced by the standard disc irradiation model with a single irradiation parameter for NUV, Optical and NIR spectral bands. NIR (NUV) band is correlated with soft (hard) X-ray flux changes during the state transition interval, respectively. In our interpreta- tion, at the moment of X-ray state transition UV-emitting parts of the accretion disc are screened from direct X-ray illumination from the central source and are heated primary by hard X-rays (E > 10 keV), scattered in the hot corona or wind possibly formed above the optically-thick outer accretion flow; the outer edge of multi-colour disc, which emits in Optical-NIR, can be heated primary by direct X-ray illumination. We point out that future simultaneous multi-wavelength observations of X-ray Nova systems during the fast X-ray state transition interval are of great importance, as it can serve as 'X-ray tomograph' to study physical conditions in outer regions of accretion flow. This can provide an effective tool to directly test the energy-dependent X-ray heating efficiency, vertical structure and accretion flow geometry in transient LMXBs.

An experimental investigation on combined sublimation and transpiration cooling for sintered porous plates

To enhance the cooling effect in stagnation regions, a combined cooling method of sublimation and transpiration through locally activating transpiration was proposed and experimentally evaluated in the hot gas wind tunnel at the USTC. This combined cooling was achieved by sublimating an impermeable coating layer of low sublimation point material. The sintered porous plate used in this experiment consists of nickel-based super alloy with a porosity of 0.33, and Teflon is used as sublimation coating material. The cooling performances of the combined cooling method were compared with traditional transpiration cooling. The experimental data indicate that: (1) the coating layer sublimation mainly happens in the upstream and provides a flexible way to locally activate transpiration cooling, which means that where heat flux is the highest, the transpiration cooling is locally activated. (2) The combined cooling can effectively solve the problem of extremely high heat flux in stagnation regions, and reduce the consumption of transpiration coolant. Under the conditions of this experiment, at a coolant injection ratio of 0.60%, the cooling effectiveness at the leading edge of the specimen is 63% higher in comparison with traditional transpiration cooling. (3) The uniformity of temperature distribution can also be achieved by adjusting coolant mass flow rate for the combined cooling.

Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

Abstract Photoevaporation is an important dispersal mechanism for protoplanetary disks. We conduct hydrodynamic simulations coupled with ray-tracing radiative transfer and consistent thermochemistry to study photoevaporative winds driven by ultraviolet and X-ray radiation from the host star. Most models have a three-layer structure: a cold midplane, warm intermediate layer, and hot wind, the last having typical speeds and mass-loss rates when driven primarily by ionizing UV radiation. Observable molecules, including CO, , and re-form in the intermediate layer and survive at relatively high wind temperatures due to reactions being out of equilibrium. Mass-loss rates are sensitive to the intensity of radiation in energy bands that interact directly with hydrogen. Comparison with previous works shows that mass-loss rates are also sensitive to the treatment of both the hydrodynamics and thermochemistry. Divergent results concerning the efficiency of X-ray photoevaporation are traced in part to differing assumptions about dust and other coolants.

Large turbulent reservoirs of cold molecular gas around high-redshift starburst galaxies.

Starburst galaxies at the peak of cosmic star formation are among the most extreme star-forming engines in the Universe, producing stars over about 100 million years (ref. 2). The star-formation rates of these galaxies, which exceed 100 solar masses per year, require large reservoirs of cold molecular gas to be delivered to their cores, despite strong feedback from stars or active galactic nuclei. Consequently, starburst galaxies are ideal for studying the interplay between this feedback and the growth of a galaxy. The methylidyne cation, CH+, is a most useful molecule for such studies because it cannot form in cold gas without suprathermal energy input, so its presence indicates dissipation of mechanical energy or strong ultraviolet irradiation. Here we report the detection of CH+ (J = 1-0) emission and absorption lines in the spectra of six lensed starburst galaxies at redshifts near 2.5. This line has such a high critical density for excitation that it is emitted only in very dense gas, and is absorbed in low-density gas. We find that the CH+ emission lines, which are broader than 1,000 kilometres per second, originate in dense shock waves powered by hot galactic winds. The CH+ absorption lines reveal highly turbulent reservoirs of cool (about 100 kelvin), low-density gas, extending far (more than 10 kiloparsecs) outside the starburst galaxies (which have radii of less than 1 kiloparsec). We show that the galactic winds sustain turbulence in the 10-kiloparsec-scale environments of the galaxies, processing these environments into multiphase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star-formation rates. Another mass input is therefore required for these reservoirs, which could be provided by ongoing mergers or cold-stream accretion. Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase of a galaxy instead of quenching it.

Entrainment in trouble: cool cloud acceleration and destruction in hot supernova-driven galactic winds

Efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds. For fiducial assumptions about the timescale for cloud shredding from high-resolution simulations (which neglect magnetic fields) we show that cool clouds with temperature from $T_{c}\sim 10^{2}-10^{4}$ K seen in emission and absorption in galactic winds cannot be accelerated to observed velocities by the ram pressure of a hot wind. Taking into account both the radial structure of the hot flow and gravity, we show that this conclusion holds over a wide range of galaxy, cloud, and hot wind properties. This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow. Given these difficulties with ram pressure acceleration, we discuss alternative models for the origin of high velocity cool gas outflows. Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud's life. For $T_{c}=10^{3}$\,K and $10^{4}$\,K clouds, we show that if conductive evaporation can be neglected, the cloud shredding timescale must be $\sim15$ and 5 times longer, respectively, than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations.

Bioclimatic Analysis in Pre‐Design Stage of Passive House in Indonesia

The objective of this study is to investigate the climate characteristics of Indonesian regions using an Olgyay Bioclimatic chart, a Givoni–Milne Bioclimatic chart and a Mahoney Table as the pre‐design stage in the development of a passive house design standard for residential house construction in Indonesia. Jakarta was the city chosen for deep analysis, because it represents most of the Indonesian regions in terms of temperature and relative humidity. The Olgyay Bioclimatic chart showed that the climate needs a high wind velocity to counteract the vapor pressure and shading in order to reduce solar gain entering the building. The Givoni–Milne Bioclimatic chart proposed natural ventilation and shading devices, while the Mahoney Table recommends open spaces for protection against hot wind, rooms single‐banked and permanent provision for air movement. The composite size of the opening at body height is better to allow the wind to counter the high levels of humidity and temperature. Heavy walls and roofs are suggested, as well as the provision of protection devices for the high amount of rainfall. Energy simulation was also done to investigate the effectiveness of the passive strategy proposed by the bioclimatic analysis. These results give a contribution as the indispensable basis for the development of a passive house standard in Indonesia.

Hydrodynamical models of cometary H ii regions

We have modelled the evolution of cometary HII regions produced by zero-age main-sequence stars of O and B spectral types, which are driving strong winds and are born off-centre from spherically symmetric cores with power-law ($\alpha = 2$) density slopes. A model parameter grid was produced that spans stellar mass, age and core density. Exploring this parameter space we investigated limb-brightening, a feature commonly seen in cometary HII regions. We found that stars with mass $M_\star \geq 12\, \mathrm{M}_\odot$ produce this feature. Our models have a cavity bounded by a contact discontinuity separating hot shocked wind and ionised ambient gas that is similar in size to the surrounding HII region. Due to early pressure confinement we did not see shocks outside of the contact discontinuity for stars with $M_\star \leq 40\, \mathrm{M}_\odot$, but the cavities were found to continue to grow. The cavity size in each model plateaus as the HII region stagnates. The spectral energy distributions of our models are similar to those from identical stars evolving in uniform density fields. The turn-over frequency is slightly lower in our power-law models due to a higher proportion of low density gas covered by the HII regions.