Predicting RF Signal Degradation Through the Hypersonic Plasma Sheath Using dsmcFoam and PIC Method

The transmission and degradation of RF signals through the plasma sheath surrounding hypersonic vehicles is investigated using a DSMC flow solver method. The OpenFoam library dsmcFoam is used to simulate flow around hypersonic bodies and obtain flow field properties to aid in calculations of signal degradation. The study of RF signal degradation and the viability of communications for hypersonic vehicles is of supreme importance for the future of aerospace as hypersonic travel and warfare is becoming technologically possible. Integrating over the output electron number density profile yields values for attenuation that drop below 100 decibels in the transmission window of 10 to 30 GHz. In this window, phase shift values remain between 2 and 10 radians. Outside of this transmission window, attenuation and phase shift are high and indicate poor chance of viable communications. If signal degradation models can be verified and improved with wind tunnel and flight data, these results suggest that vital radar and satellite communications are possible through the plasma sheath and can be decoded using accurately predicted degradation values

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Application of Direct Simulation Monte Carlo Method to Computation of RF Signal Degradation During Hypersonic Flight

In order to further understand the hypersonic blackout problem, the first step is to investigate models to quantify signal degradation and begin implementing these models to representative plasma sheath and flow data. This research is the first attempt at implementing a model to predict RF signal degradation through the plasma sheath surrounding the hypersonic air vehicle. The investigation is performed using a Direct Simulation Monte Carlo (DSMC) based flow solver. The dsmcFoam solver in the OpenFoam library is used to simulate the flow around hypersonic bodies to obtain flow field properties, most importantly the electron number density profile, to aid in the calculations of signal degradation. The study of viability of RF communications from hypersonic, fixed-wing aircraft are paramount to the future of hypersonic military capabilities and even hypersonic travel. Predicting signal degradation for a transmission along a line of sight in real-time can eliminate radio blackout by guiding the gain models on the signal decoding side and allowing reconstruction of the transmission. Even without reconstruction, accurately predicting when signals will be unrecoverable can serve as an indicator for a hypersonic vehicle to send communications to a different ground station or satellite. The integration method employed to integrate over the output electron number density profile yields values for attenuation that drop below 100 decibels in the transmission window of 10 to 30 GHz. Outside of this transmission window, attenuation and phase shift are high indicating poor chance of viable communication. If signal degradation models can be verified and improved with the flight data in the future, these results suggest that vital radar and satellite communications are possible through the plasma sheath and can be decoded using accurately predicted degradation values

Long-Term Outcomes of Diabetic Patients With Critical Limb Ischemia Followed in a Tertiary Referral Diabetic Foot Clinic

We describe the long-term outcomes of 510 diabetic patients with critical limb ischemia (CLI) and an active foot ulcer or gangrene, seen at the University Hospital of Rome Tor Vergata, a tertiary care clinic

Endovenous ablation of incompetent perforating veins is effective treatment for recalcitrant venous ulcers

ObjectivesEndovenous closure of incompetent saphenous veins has been reported to facilitate venous ulcer healing; however, there is little information about the effectiveness of perforator ablation (PA) in healing recalcitrant venous ulcers. We report our experience with PA with venous ulcers unresponsive to prolonged compression therapy.MethodsPatients with nonhealing venous ulcers of >3 months' duration underwent duplex ultrasound to assess their lower extremity venous system for incompetence of superficial, perforating, and deep veins. Patients who had either no saphenous incompetence or persistent ulcers after saphenous ablation underwent PA of incompetent perforating veins >3 mm that demonstrated reflux; initial treatment was performed on the perforator vein adjacent to the ulcer with additional incompetent veins treated if ulcer healing failed.ResultsSeventy-five ulcers with 86 associated incompetent perforating veins were treated with PA in 45 patients with CEAP 6 recalcitrant venous ulcers. Treated incompetent perforator veins were located in the medial ankle (61%), calf (37%), and lateral ankle (2%). Initial success of PA, assessed by postprocedure duplex ultrasound, was 58%; repeat ablation was 90% successful and 71% had eventual successful perforator closure. No complications (skin necrosis, infection, or nerve injury) occurred. Failure of ulcer healing with successful perforator closure occurred in 10% and was due to intercurrent illness, patient noncompliance, and patient death due to unrelated causes. Of patients who healed their ulcers, the healing occurred at a mean of 138 days; an average PA of 1.5 incompetent veins per ulcer was required for healing. Ninety percent of ulcers healed when at least one perforator was closed; no ulcer healed without at least one perforator being closed.ConclusionsThis experience demonstrates both the feasibility and effectiveness of PA for a selected group of patients with venous ulcers who fail conventional therapy with compression

Reduced primary patency rate in diabetic patients after percutaneous intervention results from more frequent presentation with limb-threatening ischemia

ObjectiveAlthough patients with diabetes are at increased risk of amputation from peripheral vascular disease, excellent limb-salvage rates have been achieved with aggressive surgical revascularization. It is less clear whether patients with diabetes will fare as well as nondiabetics after undergoing percutaneous lower extremity revascularization, a modality which is becoming increasingly utilized for this disease process. This study aimed to assess differential outcomes in between diabetics and nondiabetics in lower extremity percutaneous interventions.MethodsWe retrospectively studied 291 patients with respect to patient variables, complications, and outcomes for percutaneous interventions performed for peripheral occlusive disease between 2002 and 2005. Tibial vessel run-off was assessed by angiography. Patency (assessed arterial duplex) was expressed by Kaplan-Meier method and log-rank analysis. Mean follow-up was 11.6 months (range 1 to 56 months).ResultsA total of 385 interventions for peripheral occlusive disease with claudication (52.2%), rest pain (16.4%), or tissue loss (31.4%) were analyzed, including 336 primary interventions and 49 reinterventions (mean patient age 73.9 years, 50.8% male). Comorbidities included diabetes mellitus (57.2%), chronic renal insufficiency (18.4%), hemodialysis (3.8%), hypertension (81.9%), hypercholesterolemia (57%), coronary artery disease (58%), tobacco use (63.2%). Diabetics were significantly more likely to be female (55.3% vs 40.8%), and suffer from CRI (23.5% vs 12.0%), a history of myocardial infarction (36.5% vs 18.0%), and <three-vessel tibial outflow (83.5% vs 71.8%), compared with nondiabetics, although all other comorbidities and lesion characteristics were equivalent between these groups. Overall primary patency (± SE) at 6, 12, and 18 months was 85 ± 2%, 63 ± 3% and 56 ± 4%, respectively. Patients with diabetes suffered reduced primary patency at 1 year compared with nondiabetics. For nondiabetics, primary patency was 88 ± 2%, 71 ± 4%, and 58 ± 4% at 6, 12, and 18 months, while for diabetics it was 82 ± 2%, 53 ± 4%, and 49 ± 4%, respectively (P = .05). Overall secondary patency at 6, 12, and 18 months was 88 ± 2%, 76 ± 3%, and 69 ± 3%, and did not vary by diabetes status. One-year limb salvage rate was 88.3% for patients with limb-threatening ischemia, which was also similar between diabetics and nondiabetics. While univariate analysis revealed that female gender, <three-vessel tibial outflow, and a history of tobacco use were all predictive of reduced primary patency (P < .05), none of these factors significantly impacted secondary patency or limb-salvage rate. Furthermore, only limb-threatening ischemia remained a significant predictor of outcome on multivariate analysis, suggesting that the poorer primary patency in diabetics is related primarily to their propensity to present with limb-threatening disease compared with nondiabetics.ConclusionPatients with diabetes demonstrate reduced primary patency rates after percutaneous treatment of lower extremity occlusive disease, most likely due to their advanced stage of disease at presentation. However, despite a higher reintervention rate, diabetics and others with risk factors predictive of reduced primary patency can attain equivalent short-term secondary patency and limb-salvage rates. Therefore, these patient characteristics should not be considered contraindications to endovascular therapy

Evaluation of pressure transmission and intra-aneurysmal contents after endovascular repair using the Trivascular Enovus expanded polytetrafluoroethylene stent graft in a canine model of abdominal aortic aneurysm

ObjectiveEndotension has been defined as persistently increased pressure within the excluded sac of an abdominal aortic aneurysm (AAA) resulting in increasing aneurysm size after endovascular repair in the absence of endoleak. Devices that use expanded polytetrafluoroethylene (ePTFE) have been associated with the development of endotension and continued AAA enlargement. In this study, intra-aneurysmal pressure and aneurysm content were evaluated after endovascular repair with the Enovus ePTFE stent graft in a canine model.MethodsProsthetic ePTFE aneurysms, each containing a solid-state, strain-gauge pressure transducer, were implanted in the infrarenal aorta of 13 mongrel dogs (25-35 kg). A second pressure transducer was inserted into the native aorta for systemic arterial pressure measurement. The stent graft was then deployed to exclude the aneurysm via distal aortic access. Comparison was made among three distinct stent grafts: the Trivascular Enovus (nonporous ePTFE; four animals), the original Gore Excluder (porous ePTFE; five animals), and the Medtronic AneuRx (Dacron; four animals). Daily systemic and intra-AAA pressures were measured for 4 weeks. Intra-aneurysmal pressures were indexed to simultaneously measured systemic pressures. After 4 weeks, the aorta, the prosthetic aneurysm, and its contents were harvested, photographed, and processed for histologic investigation with hematoxylin and eosin and Masson trichrome staining.ResultsWithin 24 hours after exclusion, the mean arterial pressure and pulse pressure within the AAA sac tapered to less than 20% of systemic pressure for all three stent graft types. Throughout the postoperative period, significantly lower indexed intra-aneurysmal pressures were present in the Enovus- and AneuRx-treated aneurysms as compared with those treated with the original Excluder stent graft (0.05 ± 0.04, 0.16 ± 0.06, and 0.06 ± 0.03 for the Enovus, Excluder, and AneuRx, respectively). Histologic analysis of the Enovus-treated aneurysms demonstrated intraluminal content characterized almost entirely by erythrocytes and infrequent white blood cells without the fibrin organization—characteristics of acute or chronic thrombus. This contrasted with the content of the Excluder-treated aneurysms, which contained poorly organized fibrin deposition suggestive of acute thrombus, and of the AneuRx-treated aneurysms, which demonstrated mature, well-organized collagenous connective tissue.ConclusionsExclusion of the AAA with the Enovus stent graft resulted in nearly complete elimination of intra-aneurysmal pressure in this model. Histologic analysis of the aneurysm content further suggested complete exclusion, including elimination of circulating clotting factors and fibroblasts responsible for thrombus formation and reorganization. Ultimately, clinical evaluation will be necessary to demonstrate the effectiveness of this stent graft in preventing the development of endotension.Clinical RelevanceEndovascular aneurysm repair is an effective method for the treatment of abdominal aortic aneurysm (AAA) subjected to the unique complications of endoleak and endotension, the indirect pressurization of a sac in the absence of endoleak. In our model, AAA exclusion with the Enovus stent graft results in inhibition of fluid and serum transudation into the AAA sac, a corresponding prompt pressure decay profile, and near-complete elimination of intra-aneurysmal pressure. With the advent of implantable wireless pressure transducers, this research can be readily translated to the clinical setting. Future intraoperative and postoperative studies may help elucidate the clinical significance of pressure decay profiles in identifying successful AAA exclusion and monitoring for the development of endotension and its clinical sequelae